[FIX] account_analytic_default: compute instead of default_get + onchange

Task 2075249

The commit dd627b96 added a default_get to retrieve the default analytic
values. However, it was depending on self, which cannot be done in a
api.model

In this commit, we use an editable compute in order to update the
analytic values in all the cases, and to reduce the duplication of the
code.